This invention relates generally to an air-fuel mixture ratio control device for controlling the air-fuel ratio of the fuel mixture in a carburetor by using an exhaust gas sensor, and more specifically the invention relates to an improvement in an electromagnetic valve which serves as an actuator for air bleed control.
FIG. 1 shows an air-fuel mixture ratio control device using a conventional electromagnetic valve. An engine body 10 is operatively connected to a suction manifold 10 and to an exhaust manifold 12 on which an exhaust gas sensor 13 is disposed. A carburetor body 20 is provided in which there is a small venturi pipe 21, a large venturi pipe 22 and a throttle valve 23. A float chamber 24 communicateds via ports with the carburetor body 20. There are also formed an air inlet 25 for electromagnetic valve 40, an air bleed control port 26 for a high-speed fuel system and an air bleed control port 27 for a low-speed fuel system. A control circuit 30 is connected to a battery 31 and the exhaust gas sensor 13 and an electromagnetic valve having a valve body 40, a case 41 for the valve, coils 42 and lead wires 43 connected to the control circuit 30, and a spring 44, an armature 45, a valve 46 cooperating with a seat 47, an air inlet 48, an air bleed control port 49 for a high-speed fuel system, and an air bleed control port 49a for a low-speed fuel system. Referring to FIG. 1, when the coil 42 is energized or de-energized, the armature 45 is moved up and down by the electromagnetic attraction and by the spring 44, thereby opening and closing the seat 47 of the valve 46. The coil 42 is applied with a rectangular wave pulse signal of a constant frequency. By controlling the time ratio between "on" and "off" in the pulse signal (a duty ratio), i.e., an open-close time ratio of the valve 46, the quantity of air bleed is controlled as a function of the duty ratio.
In the conventional air-fuel mixture ratio control device in which the air bleeds for both the high- and low-speed fuel systems are simultaneously controlled by one electromagnetic valve, it is difficult to form a complete seal between the valve 46 and the seat 47 when the valve is closed because every time the valve 46 opens or closes, it rotates and the contacting portion between the vlve 46 and the seat 47 shifts as the valve rotates. The imperfect seal between the valve 46 and the seat 47 as caused by the rotation of the valve 46 not only makes it impossible to accurately control the quantity of the air bleed, but it also allow the bleed air to flow between the ports 26 and 27 of the carburetor 20 resulting in a change in the air-fuel ratio and in a worst case a stoppage of the engine during idling.